Seals for rotary or reciprocatory shafts, are well known in the art in which packing material is confined within and compressed within a sealing gland surrounding the shaft, the purpose of the packing material being to prevent seepage of fluids in a direction longitudinally of the shaft from a high pressure zone at one side of the gland to a low pressure zone, or atmosphere, at the opposite side of the gland. Further, particularly in high temperature and high pressure valve applications, such seals are required to withstand elevated temperatures, and pressures from 750 P.S.I. and up to 4200 P.S.I., commonly encountered pressures being in the range of 2800-3200 P.S.I., this requiring that the seals be highly resistive to thermal distortion or destruction. Commonly, such seals are required to withstand temperatures in the order of 900.degree. F. when exposed to ambient air, and, temperatures of the order of 1,200.degree. F. in applications handling live steam.
A multitude of different materials have been proposed as packing materials for shaft seals, including natural fibers such as cotton or wool, or inorganic fibers such as asbestos, glass fibers, rock wool, and carbon or graphite fibers, either in compacted form, or, when the form of woven materials, such materials typically being impregnated with bonding agents. While such materials can perform satisfactorily under certain conditions, invariably they are subject to one or more disadvantages such as attrition by abrasion at the rotating or axially reciprocating surface of the shaft, or, due to thermal destruction or rapid aging at elevated temperatures. Further, if the packing materials are comprised of fibrous materials, then, the interstices between the fibers permit leakage through the body of the packing material itself, thus resulting in objectionable leakage and pressure losses.
U.S. Pat. No. 3,404,061 issued Oct. 1, 1968, teaches a material for use in substitution for fibrous materials, and which is now commonly known in the art as "Grafoil". This material is an expanded graphite formed by expanding graphite particles of natural or synthetic origin by a factor of at least 80 times in the "C" crystallographic axis dimension, and then compressing the expanded particles to form a cohesive structure. A full description of the method of making expanded graphite is to be found in the said U.S. Patent.
One common usage of such material is in the formation of shaft seals by winding a length of a flexible tape formed from expanded graphite onto a mandrel to form a solid annulus of the required dimensions for filing the packing gland of the shaft, followed if desired by a pressing operation to size the formed annulus to close tolerances.
The formed annulus is then positioned around a shaft to be sealed and within the confines of the sealing gland, and is compressed in an axial direction to bring it into the required sealing relationship with the outer periphery of the shaft to be sealed.
While such seals perform admirably under favorable conditions, they are less than successful in their performance under adverse conditions, resulting from poor condition of the associated equipment, such as, scarred, galled, or pitted shafts or valves stems, particularly in situations in which they are subjected to both high temperatures and also high pressures. Such conditions, particularly at high pressures, can result in extrusion of the expanded graphite material in a direction longitudinally of the shaft, with consequential loss in the sealing ability of the seal.
Further, any eccentricity of the shaft caused by wear in its bearings can cause asperities at the interface of the shaft and seal, this resulting in particle transfer at contact points and resulting in volumetric reduction of the graphite material of the seal, again with a loss in the sealing ability of the seal.
As the seal itself is comprised by a substantially rigid and inflexible member having little resiliency, there is the further problem of the seal working or cold flowing under the forces exerted thereon by the rotating or reciprocating shaft, with the consequence of either an increase in the internal diameter of the seal, and consequential leakage, or, the inner periphery of the seal being distorted out of the round and again, promoting leakage past the seal.
It has been prior proposed in Nixon U.S. Pat. No. 4,116,451 issued Sept. 26, 1978 to incorporate a metal core member into such seals in order self energize the seals and to impart to them a degree of flexibility and resiliency, and in order to overcome the disadvantages mentioned above. Such prior proposals have proved to be only partially successful in overcoming the problems involved, in that they are of insufficient resiliency, or, they are prone to fatigue under periods of extended use.
An alternative and highly successful approach to these problems has been in the employment of seals formed from braided yarns of natural or inorganic fibers. Such materials possess all of the required characteristics of resilience and conformability, but, are hampered with several major disadvantages, particularly when used in high-pressure environments.
One of these is the lack of structural integrity of such braided materials such arises from breakage and attrition of the fibers used in the yarns from which the braided seal material is fabricated, and which results from direct frictional contact with the periphery of the shaft.
Further, and when exposed to compressive forces of 2-5 times the system pressure such as is required to establish initial sealing pressure, the fibers can become crushed and broken with a consequential loss in sealing ability of the seal even prior to the associated equipment being put into service.
Also, only a relatively small number of fibers are in actual contact with the outer periphery of the shaft, and those particular fibers must absorb all of the forces imposed on the seal by the angular or linear displacement of the shaft relative thereto.
These problems result in relatively rapid destruction of the seal, particularly in the event that the seal is used in pumps for pumping fluids having discrete particles of abrasive material entrained therein, for example, in the pumping of slurries, or in the pumping of air suspensions of abrasive particulate material.
The other problem arises from the foraminous nature of the braided packing material, which provides for leakage directly through the body of the seal, this problem limiting the use of such seals to relatively low pressure applications.